In traditional light projection systems, a condenser lens gathers light from a light source and projects a light beam in a forward direction. A spherical retro-reflector placed behind the light source redirects light toward the condenser lens that would otherwise be lost. The condenser lens attempts to evenly distribute light over an object, the image of which is projected onto a distant surface (projection screen) by a projection lens placed in front of the object.
As described in Warren J. Smith, Modern Optical Engineering, 229-230 (1990), the condenser is used to form an image of the source in the pupil of the projector lens such that the lens aperture has the same brightness as the source. An object placed at the focal point of the projection lens is illuminated by light from the condenser lens which converges upon the focal point of the condenser. The condenser's focal point should lie at the principal point of the projection lens. The projection lens is used to form an image of a brightly illuminated object upon the distant surface of a projection screen. To obtain maximum illumination at the edges of the image projected on the screen, the condenser lens must be large enough to prevent vignetting and provide sufficient magnification to fill the pupil of the projection lens.
Practically, the projection condenser system described above suffers from high losses in the light collection system owing to poor coupling between the retro-reflector and the condenser lens, and from high losses in the condenser lens. The low efficiency of this system produces a dim image on the projection screen.
To improve the efficiency of a light projection system, the condenser lens and retro-reflector are replaced by an elliptical reflector, which gathers light more efficiently by surrounding the source which is placed at a first focus of the ellipse. The elliptical profile of the reflector "condenses" light at a second focus of the ellipse thereby eliminating the need for a condenser lens. Light rays converge upon the second focus bounded by a cone having a certain half angle. The cone's apex is located at the ellipse's second focus. The area illuminated by this system is inversely proportional to the cone's half angle. While the goal of this system is to uniformly illuminate an object at the ellipse's second focus, the actual distribution of light energy is typically peaked in the center. This peak becomes more pronounced as the cone angle increases and the illuminated area becomes smaller.
Since the elliptical reflector both collects and condenses light from the source, various properties of the reflector's geometry affect the performance of the system. The eccentricity of the reflector affects the spot size and uniformity of the light distribution at the second focus. A long, "slow" ellipse reflects light into a cone with a smaller half angle for a given diameter. Such an ellipse produces a more uniform, less peaked light distribution at the second focus. Also, the reflector's shape is less sensitive to surface errors. However, a slow ellipse has a longer focal length which results in a long illumination system, and the focused spot produced by the reflector is large.
A "fast" ellipse has a shorter focal length and produces a small focused spot. However, a fast ellipse has a larger cone angle for a given diameter and yields a less uniform, more highly peaked light distribution at the second focus. Also, the reflector's shape is more sensitive to surface errors.
Various properties of the light source also affect illumination system performance in that perfectly elliptical reflectors form images of point sources only. Extended sources increase the size of the spot produced at the reflector's second focus and increase the angles of the rays exiting the reflector.
Various properties of the projection lens affect optical system performance. To form a high quality image, the lens must be matched to the size of the object to be projected. A large object requires large, expensive lenses, while a small object can be projected by smaller, less expensive lenses. A projection lens accepts and projects light rays which approach the lens within an acceptance cone having a certain half angle. "Fast" lenses have a large acceptance cone and accept light over large illumination angles. However, fast lenses have highly curved, expensive elements which are difficult to design and fabricate, and result in large optical aberrations and poorer imagery. "Slow" lenses have smaller acceptance cone angles and therefore accept light within a narrow acceptance cone. Slow lenses have more slightly curved, less expensive elements which are easier to design and fabricate, and result in better imagery.
Various properties of the reflector and projection lens in combination affect system performance. A slow ellipse illuminating a slow lens has good imagery but poor efficiency because the large spot at the ellipse's second focus overfills the lens' entrance aperture. This reflector and lens combination results in the longest overall system length.
A slow ellipse illuminating a fast lens is a poor choice since the longer overall illumination system size and large illuminated spot size of the slow ellipse are retained in this combination. Also, there is the poor imagery of the fast lens, and much of the cost of the expensive fast lens is wasted as the lens' acceptance cone is not filled by the slow reflector.
A fast ellipse illuminating a fast lens produces the shortest overall optical system size and good efficiency. However, the high cost and poor image quality of the fast lens is prohibitive. Also, the fast ellipse produces a highly peaked irradiance distribution in the lens' object plane. The result is a poor image with nonuniform illumination.
The most desirable combination is a fast ellipse illuminating a slow lens. This arrangement yields a shorter overall system size, smaller object size, and better imagery at the least cost. However, light is vignetted in the slow lens due to overfilling of its narrow acceptance cone by the fast ellipse. Also, the projected irradiance distribution is highly peaked.
A need exists to modify the angular and spatial distribution of the light collected by an elliptical reflector for projection applications. The prior art has shown a variety of techniques for accomplishing this objective. Y. Ooi et. al., "Reflective-Type LCPC Projection Display," SID Digest of Technical Papers, Vol. 26, 1995, pp. 227-230 describes an illumination system comprising an extended light source, an elliptical reflector, and a cone prism. The cone prism, as shown in FIG. 1, is a solid-glass, refracting device used to modify the angle of light exiting the reflector. While the cone prism does redirect light rays into a cone of smaller half angle, this optic is highly position-sensitive and must be placed behind the second focus of the reflector. If not, much of the light is subject to Total Internal Reflection (TIR) at the exit face and does not reach the object plane. When the optic is properly positioned, some light rays still undergo TIR. Finally, the irradiance distribution, while improved, is still not uniform.
The ProSpot luminaire made by Morpheus Lights, Inc. of San Jose, Calif., and as disclosed in the April 1994 issue of Lighting Dimensions magazine, used an arc lamp coupled with an ellipsoidal reflector to direct light toward an image plane. An internally mirrored tube reflects the outer periphery of the spray of light from the reflector back in toward the center, at once flattening the field at the image plane and increasing the efficiency of the luminaire.
Another internally mirrored tube is disclosed in U.S. Pat. No. 5,188,452. A "light mixing channel 22" is described as an elongate, longitudinally-extending, polygonal tubular structure, hollow and open at both ends, whose interior sides may be straight, tapered or curved and are mirrored or polished so as to reflect visible light rays. The light mixing channel is used, as shown in FIG. 23 thereof, to combine light rays that have been filtered through one or more color filters with un-filtered light rays so as to produce a homogeneous color wave front at the exit of the channel. But, as shown in FIG. 5 thereof, the light mixing channel of the referenced patent does nothing to alter the angle of light rays approaching a projection lens, but merely extends the length of the optical system between the second focal point of the reflector and the object plane of the projection lens.
Another technique utilizes a Fresnel converging lens positioned behind the second focus of an elliptical reflector to reduce the angle of diverging light rays so as to match the angle of a projection lens' acceptance cone. This technique yields a relatively large object size and a large overall system size, while providing only poor angle control. The Fresnel lens also does little to modify the spatial distribution of the light illuminating the lens' object plane.
A technique for limiting the angles of the rays entering a projection lens is shown in FIG. 2, and utilizes a system of apertures comprising two circular holes cut into two flat plates. Such a system of apertures is often employed in microscopes, telescopes, and other diagnostic optical instruments. This system of apertures or baffles acts as an angular filter. When the input aperture 2 is illuminated, the rays exiting the output aperture 4 are bounded by a cone whose half angle is defined by the aperture diameters and the distance between the apertures. All rays with angles greater than the allowed exit angle, .theta..sub.out, do not pass through the rear aperture. However, rays bounded by the cone with half angle .theta..sub.out pass through both apertures without striking the two plates. If .theta..sub.out matches the half angle of the projection lens' acceptance cone, these rays propagate successfully through the lens.
If the angle of the projection lens' acceptance cone is known, and the input and output aperture diameters are chosen, then the distance between the apertures is uniquely determined. From the geometry in FIG. 2, the length of this device is given by EQU L=a+a'/tan.theta..sub.out
where a' is the radius of the output aperture, a is the radius of the input aperture, .theta..sub.out is the half angle of the cone bounding the exit beam, and L is the distance between the two apertures.
The effect of a two aperture angle filter on a projection system employing a fast ellipse and a slow projection lens is shown in FIG. 3. Here, rays from the ellipse impinge on a system of two apertures that defines a cone whose half angle matches the projection lens' acceptance cone. The rays that pass through both apertures propagate successfully through the lens. However, these rays represent only a small percentage of the rays that pass through the first aperture.
The goal is to place a device between the two apertures that will modify the angles of the light passing through these apertures so that the light passing through the output aperture will propagate successfully through the projection lens. No complete solution to this problem currently exists.